These blastpipes consist essentially of a central tube through which is blown a refining agent, for example, oxygen, and at least one outer tube concentric with the first and defining with it an annular space through which is injected a protective fluid. For certain operations a number of tubes arranged concentrically are employed, which enables distinct fluids to be introduced into the converter independently of one another.
The majority of refining blastpipes, whatever their type, exhibit on the circumference of the peripheral tube an outer ring which fulfills a double function: to ensure on the one hand the sealing of the converter at the place of insertion of the blastpipe, and to enable on the other hand the securing of special claws for extraction of the blastpipe out of the converter in case of need.
This ring is generally connected by welding onto the circumference of the outer tube. The result of this is in general a deformation over the inner wall of the outer tube at the level of the ring. This deformation is characterized by a slight swelling causing a local contraction of the inner diameter of the tube.
In the case of multiflow blastpipes this contraction affects the geometry of the annular gap and consequently prejudices the operation of these blastpipes, particularly of those having an annular gap of very small thickness.
The usual solution consist in subsequent re-machining of the inside of the tube. It is clear that such a supplementary intervention cannot but burden the cost of manufacture of the blastpipes.